Cesium聚簇

上篇中 已经出现了 很多图元的时候 性能优化的事 ，但是地图上 会有密密麻麻的各种图标 或者文字

这就要用到 聚簇了

 	var dataSourceForCluster = new Cesium.CustomDataSource('cluster');
    var count = 100000;
    for (var i = 0; i < count; ++i) {
        dataSourceForCluster.entities.add({
            id: "" + i,
            position: new Cesium.Cartesian3.fromDegrees(Math.random() * 100, Math.random() * 100, 10000),
            billboard: {
                image: '../src/assets/image/wx.png',
                height: 30,
                width: 30
            }
        });
    }
    var dataSourcePromise = viewer.dataSources.add(dataSourceForCluster);
    dataSourcePromise.then(function (dataSource) {
        var pixelRange = 15;
        var minimumClusterSize = 3;
        var enabled = true;
        dataSource.clustering.enabled = enabled;
        dataSource.clustering.pixelRange = pixelRange;
        dataSource.clustering.minimumClusterSize = minimumClusterSize;

        var pinBuilder = new Cesium.PinBuilder();
        var pin50 = pinBuilder.fromText('50+', Cesium.Color.RED, 48).toDataURL();
        var pin40 = pinBuilder.fromText('40+', Cesium.Color.ORANGE, 48).toDataURL();
        var pin30 = pinBuilder.fromText('30+', Cesium.Color.YELLOW, 48).toDataURL();
        var pin20 = pinBuilder.fromText('20+', Cesium.Color.GREEN, 48).toDataURL();
        var pin10 = pinBuilder.fromText('10+', Cesium.Color.BLUE, 48).toDataURL();

        var singleDigitPins = new Array(8);
        for (var i = 0; i < singleDigitPins.length; ++i) {
            singleDigitPins[i] = pinBuilder.fromText('' + (i + 2), Cesium.Color.VIOLET, 48).toDataURL();
        }
        dataSource.clustering.clusterEvent.addEventListener(function (clusteredEntities, cluster) {
            cluster.label.show = false;
            cluster.billboard.show = true;
            cluster.billboard.id = cluster.label.id;
            cluster.billboard.verticalOrigin = Cesium.VerticalOrigin.BOTTOM;

            if (clusteredEntities.length >= 50) {
                cluster.billboard.image = pin50;
            } else if (clusteredEntities.length >= 40) {
                cluster.billboard.image = pin40;
            } else if (clusteredEntities.length >= 30) {
                cluster.billboard.image = pin30;
            } else if (clusteredEntities.length >= 20) {
                cluster.billboard.image = pin20;
            } else if (clusteredEntities.length >= 10) {
                cluster.billboard.image = pin10;
            } else {
                cluster.billboard.image = singleDigitPins[clusteredEntities.length - 2];
            }
        });
    });